Apparent Wind Calculator

True wind vs apparent wind

True wind is the wind you feel when the boat is stationary, the actual meteorological wind. Apparent wind is the wind you experience while moving: the combination of true wind and the headwind created by the boat’s own forward motion.

When sailing, the sails respond to apparent wind, not true wind. Understanding the difference is critical for correct sail trim, tacking angles, and racing tactics.

The effect of boat motion

When a boat moves forward, it creates a headwind equal to its speed. This headwind adds to the true wind vectorially:

• On a beam reach (90° true wind angle), a boat doing 6 knots into a 12-knot true wind produces an apparent wind that is both stronger and forward of the true wind.
• Upwind, apparent wind is stronger than true wind.
• Downwind, apparent wind is weaker than true wind.

The formula

Apparent Wind Speed (AWS) = √(Vt² + Vs² + 2 × Vt × Vs × cos(β)) Apparent Wind Angle (AWA) = arctan(Vt × sin(β) ÷ (Vs + Vt × cos(β)))

Where:

• Vt = True wind speed
• Vs = Boat speed through water
• β = True wind angle (0° = dead ahead, 180° = dead downwind)

Practical implications

Apparent wind always shifts forward of true wind. This is why close-hauled boats sail to the wind they feel, not the true wind.

The consequence downwind is the one that catches people out. When a boat accelerates, say it picks up a wave and surfs, the apparent wind moves further forward and the apparent angle gets smaller, so the correct response is to sheet in, not ease. Slow down again and the apparent wind swings aft and you ease back out. Foilers and asymmetric-spinnaker boats live in this loop constantly, trimming to a wind angle that changes with their own speed rather than with the weather.

There is a crossover angle where apparent and true wind speed are equal. It sits at cos(β) = −Vs ÷ (2 × Vt), so for a 6-knot boat in 12 knots of true wind it falls at about 104°. Sail tighter than that and the apparent wind is stronger than the true wind; sail deeper and it is weaker. That crossover moves aft as the boat gets faster, which is why a fast multihull can feel a building breeze while bearing away.